1. Field of the Invention
This invention relates to kinetic energy absorbing devices, and more particularly to pads having a core which, under the force of an impact load, is adapted to undergo stepwise deformation, thereby to reduce significantly the peak dynamic load sustained by the pad.
2. Description of the Prior Art
Nuclear energy plants, nuclear fuel processing plants as well as other process plants incorporate pipes and conduits for conveying fluids under a broad range of pressures. Of particular concern are the extremely high pressure conduits. Should a fracture occur in such a conduit, particularly adjacent to a conduit elbow, the issuing high pressure fluids produce a jet force which whips the broken conduit at an extremely high velocity. An enormous impact load is applied by the whipping conduit to the first stationary object in its path. Absorption of the kinetic energy of such high velocity conduits is achieved by devices known as pipe whip restraint pads. The pad incorporates a core which is crushed by the impact load. Absorption of the kinetic energy is achieved by crushing, that is wrinkle buckling the core elements.
Energy absorbing honeycomb structures are known in the art, see for example, U.S. Pat. No. 3,130,819 (A. C. MARSHALL); U.S. Pat. No. 3,552,525 (C. R. SCHUDEL).
Conventional honeycomb exhibits a uniform energy absorbing characteristic when mechanical forces are applied to the columnar ends of the honeycomb cells. Generally, a honeycomb structure comprises plural corrugated ribbons of sheet material such as metal foil, paper, plastic or the like which are secured together at spaced node points. The resulting structure presents plural hollow, multisided, parallel cells. The application of mechanical forces to the columnar ends of the cells causes the cell walls to fold into small accordian-like pleats resulting in compression of the structure and absorption of energy.
Another characteristic of honeycomb is that its compression or columnar strength is considerably greater than its uniform crush strength. For this reason extremely high initial peak loads are required to initiate buckling of the cell walls. When conventional honeycomb is used as the core of a pipe whip restraint pad, the structural framework or the support to which the pad is secured also must be capable of sustaining the high peak loads.
To eliminate the high buckle-initiating peak loads, the honeycomb core has been partially crushed in a direction parallel with the cells and to a selected depth prior to being assembled into the device, see MARSHALL patent, supra. Since buckling of the core has been initiated, only a relatively low peak load is attained when the pad sustains an impact load. That is, a peak load sufficient only to continue crushing the core.
Although high buckle-initiating peak loads are not encountered by the MARSHALL core when in use, they are encountered during manufacture of the core, that is when precrushing the core. It will be appreciated that core precrushing requires the expenditure of large amounts of costly energy.
Honeycomb cores providing gradually increasing energy absorption also are known in the art, see for example the SCHUDEL patent, supra. Such honeycomb cores have a wedge-shaped end. The anvil--the member which compresses the core--encounters increasing resistance since it must collapse ever increasing cross-sectional areas of honeycomb. Wedge-shaped energy absorbers, when compressed, produce angularly presented splaying forces which cause delamination of the honeycomb at the bonded node points. The angular splaying forces are avoided in the SCHUDEL structure by providing a suitably shaped concavity in the anvil. Wedge-shaped energy absorbers may be formed from an expanded honeycomb structure presenting hexagonal cells or as corrugated spiral wound constructions.